Add -verbose:gc option for test runs

Motivation:

While investigating the recent CI machine crashes, I observed that the
JVM processes spawned by surefire sometimes take up to 1 GiB RAM.
Consuming large amount of memory isn't really a problem, but we need to
make sure no GC trashing is occuring during the tests.

Modifications:

Add -verbose:gc option to the test JVM arguments

Result:

We can determine if there is any GC anomalies going on in our CI
machine.

pom.xml

@@ -81,6 +81,7 @@
           -XX:+OptimizeStringConcat
           -XX:+HeapDumpOnOutOfMemoryError
           -Dio.netty.leakDetectionLevel=3
+          -verbose:gc
         </test.jvm.argLine>
       </properties>
     </profile>
@@ -147,6 +148,7 @@
       -XX:+UseFastAccessorMethods
       -XX:+OptimizeStringConcat
       -XX:+HeapDumpOnOutOfMemoryError
+      -verbose:gc
     </test.jvm.argLine>
   </properties>